Volume control system



Feb. 12, 1935. R. s. GLASGOW VOLUME CONTROL SYSTEM 2 Sheets-Sheet 1 Filed May 1, 1929 IIIIIII IIIIIIIII NIH- 1.4

IIIIIIII I I I I I I I I I 3 I I I I I I I I Feb. 12, 1935.

R. S. GLASGOW VOLUME CONTROL SYSTEM Filed May 1, 1929' 2 Sheets-Sheet 2 Patented Feb. 12, 1935 UNITED STAT VOLUME OONTROL SYSTEM: v Roy'SQGlasgow, St. Louis, Mo., assig non by mesne g assignments, to Radio Corporation of America, New York, N. Y.,'a. corporation of Delaware 1 Application May-1,1929, Serial N6. 359,53

1 4 Claims. (Cl. 179-471) This invention relates to a volume control system for vacuumtube amplifiers; My invention also involves a novel method of volume control for vacuum tube amplifiers. g V 1 An objectof the invention is to devise a system for controlling the amplification :or gain .of a ,vacuumtubeamplifier with a minimum distortion 7 of the amplified current.

It has heretofore. been proposed to vary. the amplificatiomor gainof vacuum tube amplifiers by means of a variable resistance inserted in ,the'plate circuit. The principal objection to this systemof volume control-is that a very large resistance is required to reduce the volume to substantially zero. For example, when this systemisapplied to ordinary broadcast. receivers :the maximum valueof the resistance required to reducethe signal froma strong local station to substantially zero volume may be inexcess of 10,000,000ohms; l v

An object of my invention is to devisea volume .control system in which a variable-resistanceinserted" in the plate circuit simultaneously serves tocontrol therplate current and to vary the biasingzpotential' applied -.,to the grid element of the amplifier. The resistance is inserted in a common path of the grid and-plate circuits, whereby as the resistance increases to decrease the plate 'current,,,the grid biasing potential isrendered more negative, which also tends toreduce: the

plate currents] Thus,zthe resistance is doubly ef: fective in controlling the plate current. By such 'an arrangement I :am enabled to obtain full range of volume variation from zero to a. maxi-- mum" witha resistance having a value: of -only 'afraction of that r'equired when the resistance serves, to control the plate current only; T l v My invention is illustrated in the accompanying drawingslin which; I

Figure 1 illustrates my invention as applied to l Figure. 2 shows the volume-control system ap-j plied to a two stage'amplifier employing vacuum tubes having uni-potential cathodes;

5" Figure 3 shows the volume control system applied to a radio receiver; and V Figure 4 is a graph, with curves showing the variation in plate current andgridrbiasing potential with changes in the volume control resistance.

5 Referring to Figure l '1, I, have showntwo transformer-coupled amplifiers A1 and A2."v T1,.T2 and T3 indicate, respectively, the input transformer, the coupling transformen and the output transformer. Amplifier tubes A1 and A2 may be either of the filamentary cathode, type or ofythe .cuits of tubes A1 and A2.

uni-potential cathode type. The cathodes of amplifiers Al and A2 are connected in parallel and to one terminal .of a volume controlresistance-Roc.

. The other terminal ofthevolume control resistance. is connected-to the negative terminal of asuitable source vof plate current Be. ,The positive terminal of the'source ofplate current is connected to the primary windingsoftrans-p formers T2 and; T3v to vcomplete the plate cirings of transformers T1' and T2 are included in the input circuitsof amplifiers of A1 and A2, respectively, but. instead of being connected directly to the cathode in the-usual manner, the common'cathode return terminalgof ,theinput circuits is connected around the volume control Operation of Figure 1 1sv as follows: From an'inspectionyof the diagram; it will be seen that the ,volume. control resistance R00 is; included in the plate circuit ofeach amplifier, andis alsoincluded in eachinput circuit. P The drop in potenon produced by theplate current flowing through the volume controlresistance is of such polarity that the grids :ofthe amplifiers are maintained negative with respect to the cathodes, the value The secondary wind- 10 resistance R'uc so asnto includethis resistance of the biasingpotential depending upon the value f of the total plate current'and-the value of the volume control resistance; :Increasing the volume control resistance, increases the externalyresistance of theplatecircuits, and, therefore, flattens I out the plate current characteristic and reduces theamplification'or gain of the amplifier. The plate current is not only reduced by thedirect action of the increasedvolume control resistance,

"but is alsoifurther reduced by the increased negative biasing potential-applied to the grids of the amplifiers. The circuit constants are-such" that with increase in volume control-resistance Rvc,

the negative biasing'potentialdueto the drop across .the volume'control.resistance increases continuously over the working range of the volume control resistance. It will'thus' be seen that the volume control resistance, when placed in .the position shown, is doubly effective in reducing the plate current, and for this reason a relatively small resistance may be employed as compared with the resistance which would be required to reduce the plate current to the same extent when such resistance is not included in the grid circuits of the amplifier. This feature will be explained in more detail hereinafter in connection with Figures 3 and 4.

In Figure 2 the invention is illustrated in connection with a two-stage amplifier employingvacuum tubes of the uni-potential cathode type in which the heater elements of the tubes are heated from an alternating source of current, and in which the plate circuits are supplied with rectified alternating current. In this figure, elements corresponding to similar elements in Figure 1 are indicated by the same reference characters. The circuit diagram is very much like that shown in Figure 1. The uni-potential cathodes K1 and K2 are connected together and to one terminal of the volume control resistance R110. The heater elements H1 and 'H2-are connected in' parallel, and are supplied with heating current from the secondary of transformer T4 connected to an alternating current supply line AC. The midpoint of the secondary winding of transformer T4 is connected to ground. The plate current for the amplifier tubes is supplied from a rectifier Rae connected to the alternating current line AC, and the circuit is otherwise the same as illustrated in Figure 1. W m

The operation of Figure 2 will be apparent from the description of the operation of Figure 1.

Figure 3 illustrates my invention as applied to a commercial radio receiver. The radio receiver comprises a tuner compartment A, an amplifying and detecting compartment B, and a compartment C containing the audio frequency amplifiers and the power supply units. Compartments A, B and C comprise metallic casings which are con.- nected together by suitable bonding connections indicated at (a) and (b).

l The tuning compartment A contains suitable tuning apparatusfor selecting the desired wave, which apparatus forms no part of the present invention, and is not described in this application. Compartment B contains a radio frequency amplifier comprising five transformer-coupled vacuum tube amplifiers A1, A2, A3, A4 and A5, and a vacuum tube detector D. The vacuum tubes employed are of the uni-potential cathode type with heater. The uni-potential cathodes of theamplifiers are connected to a common connection 2 which is grounded to the metallic casingof compartment B as at through a connection including a fixed resistance R0 in compartment C and the volume control resistance R110. The. cathodereturn terminals of the input circuits of the amplifiers are connected to a common connection 3 which is connected to the metallic casing of compartment B as indicated at (c). The heaters of the amplifier and detector tubes are supplied with heating current from secondary winding 5 of transformer T5. The midpointof secondary winding 5 is grounded to the metallic casing C as indicated at (d) The plate'current supply source for the amplifier and detector tubes includes a fullwaverectifier tube RT, the filament of which rent supply, is grounded tothe metallic casing C as indicated at (e). The midpoint of winding 6, which constitutes the positive terminal of the rectifier, is connected to a filter unit including choke coils 8 and 9, resistance and 11, and condensers 12, 13 and 14. Choke coils 8 and 9 and resistance 10 are included in series in the positive side of the plate supply circuit, while resistance 11 and condensers 12, 13 and 14 are connected in shunt to the plate supply circuit as shown. The platecircuit for the radio frequency amplifiersAl to A5.,is completed from the positive side of the rectifier filter through wire 15 to winding 16 of the loud speaker LS, and through winding 16 to a common wire 4, to which the plate circuits of the-radio frequency amplifiers are connected. The plate circuit of the detector D is completed from positive terminals 17 of the rectifier filter through primary winding of transformer T6, through radio frequency choke coil RC, to the plate-element-of the detector. The cathode of detector- Dis grounded to the metallic casing C by a connection including biasing resistance R2 which is connected to the frame of a jack J mounted on' metallic compartment C. A by-pass condenser C3 is connected between the plate and cathode elements of detector D to by-pass the radio frequency current, and a by-pass condenser C10 is connected from the detector cathode to the conductor 3 and to the casing B by connection C. The cathode-return terminal of the input circuit of detector D is grounded to metallic compartmentB and to wire 3 through normally closed jack contacts 18and 19. The midpoint of the secondary winding of transformer T6 is grounded to metallic casing C, and the end'terminals of the winding are connected, respectively, to the grids of a push-pull amplifier A6A7. The cathodes of amplifiers A6 and A7 are heated by current supplied from winding 20 of transformer T5. The midpoint of winding 20 is grounded .to casing C through abiasing resistance 21. The plate elements of amplifiers A6 and A7 are connected respectively to opposite terminals of the primary winding of transformer T7, and plate current is supplied to these amplifiers by a connection 22 from between choke coils sand 9 on the rectifier filter to the midpoint of the primary winding of transformer T7; The secondary of transformer T7 isconnected to the operating winding 23 of loud speaker LS. A resistance R3 shunted by a condenser C4 is connected from one terminal of magnetizing winding'lfi'of loud speaker LS to metallic casing C for the purposeof supplying additional magnetizingcurrent to winding 16 over that supplied by the plate currents of amplifiers A1 to A5.

' 24 indicates a plug the leadsof which are connected to a phonograph pick-up device whereby the audio frequency part of the radio receiver may be employed for the electric reproduction of phonograph records. The sleeve of plug 24 is adapted to make connectionwith a jack contact to connect resistance R5 in parallel with resistance R2, tothereby place a proper biasing voltage upon the grid of detectorD for operation as an amplifier. The tip of plug 24 makes contact with spring contact 18 of jack J to complete the grid circuit of detector D through the phonograph pick-up device and to open contact between contacts 18 and 19. With this arrangement detector D is in proper condition to function as an amplifier for supplying amplified phonograph reproduction to the input of thepush-pull amplifiers A6A7.

-- .By-pass'condensers'Ci C6 and C7 are provided to preventundesirable couplings between conductors 2, 3 and 4. High-frequency choke coil 25 is provided to prevent undesirable coupling between the amplifi'e rs'orltppdsite'sides' of 'the icon. I

By-passfcondensersfC8 andow rs "connectedin series across the filament heater energizing circuit of the amplifier, 'and the midpointor these condensers" is connected to conductori 3, which is grounded, for-the purpose of preventing undesired coupling of highirequency currents by way of the filament energizing circuit- '26-indicates"a lamp energized from -the heater circuit for illu minating the tuner dial and for indicating when shown in Figure 3, it will beseen that insofar as I the volume control feature is concerned, the diagram is the same as that shown in Figure 2, except that there are five amplifier tubes employed instead of two, and there is a fixed resistance Rc connected in series with volume control resistance Rvc. When the arrangement is operated as a radio receiver, plug 24 is withdrawn from jack J, and a high negative biasing voltage is impressed on the grid of detectorD by the drop in potential across resistance R2. With the volume control resistance Rvc adjusted to maximum volume, that is, all the volume control resistance out out, the drop in potential acrossresistance Rc impresses a proper negative biasing potential upon the grids of amplifiers A1 to A5. Upon adjusting the volume control resistance to decrease the volume from maximum, the biasing potential impressed upon the grids of the amplifiers is determined by the voltage drop across thecombined resistance R0 and Rec. The efiect of increasing the volume controlresistance is the same as explained above in connection with Figures 1 and 2, that is, it acts to directly decrease the plate current in the amplifiers, and also indirectly to decrease the plate current by increasing the negative grid bias.

For the purpose of showing the effectiveness of 'my "system of volume control, the curves shown in Figure 4 have been reproduced fromactual test of the volume control as applied to the radio receiver shown in Figure 3.

I Curve 1 indicates the variation in total plate current for the amplifiers with variations in the biasing resistance, that is, in the combined resistance of R0 and Eve. Curve 2 indicates the change in biasing potential with changes in biasing resistance. Curve 3 is reproduced to show the difference between my system of volume control and the prior system wherein the volume control resistance affects merely the value of the plate current, and the grid bias remainsconstant. Curve'3 was obtained by removing resistances Re and Rec from the position shown in Figure 3 and inserting them in the common plate circuit of the amplifiers at the point indicated at X, and in the place of resistance Re and R110, a biasing battery of a voltage equal to the drop across resistance Rc with all the volume control resistance cut out was inserted to maintain'the grids at a constant negative potential with respect to the cathodes. In the particular set tested, the value of R0 was of the order of 120 ohms, and the maximum value of R'uc was substantially 50,000 ohms. At full volume setting the total plate current was of the order of 25 milliamperes and the grid biasing voltage was of the order of 3 volts. ,The solid portion of the curves indicate the variations up to 6,000 ohms, and the dottedportion of the curves indicates the variation on a reduced scale from 6,000 to'50,000

ohms, or' to'the'minimum volume positioni 'llrom arr-inspection of the curves,it will be seen thatthe grid biasing potential constantly increases with increasing volume control resistance; at full volume the'negative bias is of the order of3 volts,

and at volume the negative bias is of theorder of 15 volts; The platecurrent decreases continuously from a maximum 'ofthe order of:25

milliamperes'at'maximum volume to a minimum of the order of 0.3 of a milliampere. As shown by curve 3, with the volume control resistance connected so as to vary the plate current alone, and

with the grid biasing potential held constant at substantially 3 volts, the plate current varies from a maximum of substantially 25 milliamperes to a minimum of substantially 3 milliamperes.

In the reception of a strong local station using my system of volume control, comfortable volume obtained with the control set at about 6,000 ohms. The signal completely disappeared at about13,000 ohms, long before the full value of the volume control resistance had been inserted in the circuit,

whereas with the volume control resistance insorted in the position represented by curve 3, when the full value of the volume control resistance had been included in the plate circuit, the signal strength was reduced only slightly below the normal volume reception of the station obtained by my system with only 6,000 ohms inserted.

From this one example, itwill be seen that a resistance of 6,000 ohms in my volume control is more effective than 50,000 ohms in the prior system. A distinct advantage is thereby gained since it is very diflicult to manufacture variable resistances of high values with any degree of uniformity and constancy.

While I have givenabove certain numerical values solely for the purpose of illustrating the operation of my invention, it will be understood that the invention is not to be limited in its application to any particular constants. It will also be understood that my system of volume control may be applied to either high or low frequency amplifiers.

What I claim is:

I. In an amplifier, an electron device having at the other end to said remaining end of the resistance and means to vary saidresistance to simultaneously vary in the same sense the internal and external impedances of said electron gevice to doubly control the volume of the ampli- 2. The method of controlling the volume of an electron device having a control electrode and an anode-cathode circuit containing a source of current and internal and external impedances which consists in varying the external impedance and simultaneously varying the internal impedance to doubly control the current in said anode-cathode circuit.

3. The method of controlling the volume of an electron device having a control electrode and an anode-cathode circuit containing a source of current and internal and external impedances which consists in varying the said external impedance and simultaneously applying to said control electrode a negativevoltage drop across a portion of said external impedance to doubly control the current in said anode-cathodecin cuit.- i r 4. In an amplifier, an electron device havinga cathode, ananode and a. control electrode, a. re.- sistance having one end connected to said cathode, a. condenser shunting said resistance, an output circuit containing a. source of directcurrent potential connected at one end to said anode and at the other end to theremaining end of said resistance, and an. input circuit connected at one end-t0 said control electrode and at the other end-toseid remaining end of the resistance, and

moons to vary said resistance to simultaneously vary in the some sense the internal and external impedancos of I said electron device to doubly control' the vohxme of the amplifier.

1 ROY S. GLASGOW 

